The invention disclosed herein pertains in general to attenuators and in particular to a multiplicity of switchably activated L-type pads coupled together to form a network having selectable attenuation settings. Such attenuator networks which have precision graduated settings are commonly known as prescription attenuators.
Prescription attenuators find a variety of uses in the instrumentation field to establish precision voltage references and in the telecommunication field where, for instance, amplifiers are configured with prescription attenuation feedback networks to produce selectable gains for compensating the loss due to various telephone line lengths. In this manner, a single amplifier unit having a plurality of switchable gain settings may be utilized to match any one of a plurality of lines. In addition, if the line characteristics change because of replacement or degradation, a craftsman need only to select a new amplification setting rather than experiment with the gain of the amplifier to arrive at a proper level.
Prescription attenuators used in conjunction with amplifiers typically provide for manually setting the amplifier amplification by way of switches mounted in the amplifier front panel. The switches, traditionally of the double-pole type, connect sections of a constant resistance T-pad between the amplifier input and output to provide amplifier gains in proportion to the attenuation of the activated section. T-pad attenuators, having three resistive components are well known in the art. A plurality of T-pads are generally cascaded and the resistive components so chosen, that each T-pad section may be switchably connected into the feedback path. An overall amplifier gain results which is the product (db sum) of the gains attributable to each individually activated section.
The versatility of a prescription gain amplifier is enhanced in view that a craftsman may select any permutation of switch settings to obtain a desired gain. When utilized with T-pad sections this added versatility is not without the attendant disadvantage of having to extend at least three wires per double-pole switch to the module faceplate or facade where the switches are physically located. In a typical 24 db gain amplifier having 0.1 db resolution the number of interconnections for eight switches will normally be twenty-five but may range as high as thirty-two (depending on the number of common connections as will be discussed below). Recognizing the cost, labor and materials involved, attempts have been made to obviate the multiple interconnection disadvantage by locating the attenuator components on the faceplate. While this measure alleviates some of the problems, other difficulties, such as trouble shooting defective circuitry, arises out of this attempted solution.
The T-pad network configuration, when used in conjunction with amplifiers to produce prescription gain does exhibit a constant input and output impedance, but such a configuration has the offsetting disadvantage of producing a six db gain when all sections are in the nonactivated state. To overcome this aspect, additional loss is inserted either before or after the amplifier stage.
Accordingly, in attenuator networks, it would be highly desirable to substantially reduce the number of interconnections and the number of attenuator components (and thus cost), yet combine the feature of faceplate-mounted switches wired to the components located on the module board. With the foregoing in mind, it is a primary object of the present invention to provide an attenuator configuration having a plurality of coupled L-pad sections to form an attenuator network wherein each section may be switchably activated by way of a single-pole type switch thereby reducing the number of switch-to-front panel interconnections by approximately 50%.
It is a further object of the present invention to provide an attenuator arrangement wherein the section impedance ratios determine the criticality of the attenuator performance rather than the individual component values. The advantage of resistor impedance ratio matching lends itself well to the attendant advantage of thick film manufacture.
It is still a further object of the present invention to provide an attenuator arrangement having approximately 30% fewer components, thus requiring less space and resulting in a reduction in cost.
As a corollary to the foregoing, it is an object of the present invention to provide an attenuator arrangement, which when used with an operational amplifier to produce prescription gain, produces zero db gain with all attenuator sections nonactivated.
These as well as other objects and advantages will be more fully appreciated by referring to the detailed description which follows hereinafter together with the appended drawings.